Ahh, the perennial problem: what to do with the pesky fact that the year isn’t exactly as long as we want. The Romans initially handled this by having the season of “winter” be of varying length. Of course, those early Romans weren’t exactly known for their accurate record keeping: the pontifex maximus (Calendar Guy) would add an extra month from time to time to keep things sort of on track, but since a calendar year was also the same as the term of office in politics, he would also add days to a year when his allies were in power and subtract them when his opponents were in charge. In other words, it pays to be on good terms with the person in charge of the calendar.
So we probably shouldn’t turn to the ancient Romans for calendar advice, though to be fair Julius Caesar is the one who authorized giving the calendar a much-needed overhaul and began the regular adding of leap days by having a second February 24 every 4 years (seriously: February 29 didn’t happen for more than a thousand years after that).
But an extra day every 4 years isn’t exactly right either: you’re off by a day every hundred or so years. So with the Gregorian calendar the leap day is skipped at the turn of a new century. Well, it’s skipped 3 out of 4 centuries: 1700, 1800, 1900, 2100, etc. skip their leap days, although 2000 (and 1600 and 2400) still have them. And that got the alignment mostly on track, but it’s not perfect.
The problem is that the imperfection is inconsistent. The length of a year actually varies a bit: According to The Guardian:
The snag is that the rotation of the Earth is not so reliable. It is gradually slowing down and factors such as disruptions in the Earth’s core, extreme weather, volcanic eruptions and earthquakes can all influence the precise length of the astronomical day. From time to time, the rotation-based clock — UT1 time — and UTC [Coordinated Universal Time] need to be brought back into line.
So occasionally, like tonight, an extra second is added [or in theory subtracted, although that's never actually happened]. Really, it’s like a Roman Intercalary month second since it doesn’t happen on a regular or predictable fashion. And for most people it means nothing except perhaps an extra kiss at midnight, but people who pay very close attention to time get some extra work in by making sure that everything is aligned perfectly world-wide.
And that causes its own problems, because what with new-fangled technology like computers, being off by a second requires all sorts of overtime and problems. The extra second is actually added at midnight Greenwich Time: according to a bulletin from the U.S. Navy here, the official sequence will be
31 DEC 2008 23 HOURS 59 MINUTES 59 SECONDS 31 DEC 2008 23 HOURS 59 MINUTES 60 SECONDS 01 JAN 2009 00 HOURS 00 MINUTES 00 SECONDS
but that’s actually 7pm here in New York and is mid-day in other parts of the world. In other words, if you’re working 9-5 in California, you might want to ask for that extra second in overtime pay.
Note 1: Did you notice that UTC, not CUT, is the abbreviation for Coordinated Universal Time and assume that the abbreviation must be from another language? If so, you’d be wrong but for an amusing reason: according to the National Institute of Standards and Technology, the abbreviation was specifically chosen not to stand for anything. The English Coordinated Universal Time would be reduced to CUT and the French Temps Universel Coordonné would be abbreviated as TUC, so the International Telecommunication Union decided that everyone in the whole world should use the initials UTC as a compromise.
Note 2: The photo of the Times Square Ball is actually from last year, and is licensed under Creative Commons Attribution 2.0 by Clare Cridland. But if you want to be current, Dave Richeson has a really cool post about the 2009 Times Square New Year’s Eve ball here on his blog Division by Zero.